Use a soft measuring tape or a strip of paper and a ruler
Make sure the penis is fully erect for an accurate measurement
Wrap the tape around the thickest part of the shaft
Keep the tape snug but not tight
Read the measurement where the tape overlaps
If using paper, mark the overlap point, then measure that length with a ruler
Measure at the middle of the shaft if you want a standard reference point
Repeat the measurement once or twice to confirm consistency